A number of catalysts have been suggested to convert engine exhaust gas components like carbon monoxide, hydrocarbons and nitrogen oxides into other gases. The first two are desirably oxidized to water and carbon dioxide while the nitrogen oxides present in the exhaust gas, generally nitric oxide, are desirably reduced to nitrogen gas. These so called "three-way" catalysts achieve simultaneous efficient (conversion &gt;80%) removal of carbon monoxide, hydrocarbons, and nitrogen oxides when the fuel mixture of an internal combustion engine is slightly "rich" in fuel, i.e., in a narrow A/F ratio range between about 14.7 and 14.4, and the exhaust is slightly reducing. Generally, these catalysts comprise a precious metal like platinum carried on an alumina support material.
Such three-way catalysts are not efficient, however, in the reduction of nitrogen oxides when engines are operated on the lean (reduced fuel) side where the A/F ratio is greater than 14.7, generally 19-27, and the exhaust gas is richer in oxygen. It is desirable, however, to operate engines on the lean side, at least part of the time, to realize a benefit in fuel economy, estimated to be in the range of 6-10%.
Zeolite based catalysts have been suggested for lean-burn operation. These zeolite based catalysts, as compared to alumina based catalysts, need to be placed further away from the engine since zeolite is not as durable as alumina under the high operating temperatures near the engine. As a result, these catalysts take longer to reach operating temperatures and hence are less efficient during cold start than three-way catalysts.
U.S. Pat. No. 5,399,324 commonly assigned with the present invention, describes a two-stage catalyst system useful for lean burn engines where the exhaust gas is exposed to the first-stage of tungsten on alumina and then to the second-stage of noble metal. It would be desirable if a durable catalyst were available which did not involve two-stages of catalysts and had good nitrogen oxide conversion efficiency under lean burn conditions. The present invention catalyst provides such a catalyst.